Even in Freshwater, Sacrificial Anodes Matter
Published: Saturday, January 30, 2021
By: By Dan Armitage
Because your boat is either on the trailer or in dry storage this time of the year, winter is a good time to check your boat's sacrificial anodes—commonly referred to as "zincs"—and replace them if necessary. Most sacrificial anodes are placed by the manufacturer in designated spaces on the engine's middle and/or lower unit, and replacements are available at your local dealership. But before you go ahead and select a set of "zincs" based solely on the brand of engine you own, it pays to be more aware of what exactly sacrificial anodes do and the available types.
What do anodes do?
Anodes create a slight electrical current by establishing a difference in electrical potential between metals. In freshwater or salt, electrons eat away at metals such as your sterndrive, prop shaft, or raw water through-hull. Anodic metals such as zinc attract and consume the corrosive electrons to protect more valuable metals from corrosion, sacrificing minute pieces of the anode as they do so. As such, sacrificial anodes need to be replaced on a regular basis, which is a DIY job as they are usually bolted into an accessible place on the engine. If the surface of your anodes is crumbled, deeply pitted, or you can tell that more than half of the chunk of metal is missing, it's time to replace them.
How to Choose a Sacrificial Anode
Boaters refer to any anodes as "zincs" because that is the metal most often chosen for the task. However, there are anodes made of other alloys such as magnesium and aluminum that may be a better choice than the traditional zinc. The choice depends on the type of metal you need to protect, the water in which the metal is immersed, and the voltage of the anode when referenced against a standard electrode made of silver/silver chloride. The higher the anode's voltage, the more easily it sheds corrosion-fighting electrons.
At a glance, you might conclude that, since magnesium has the highest negative voltage, it's the best anode to use to protect any metal on any boat. But that's not the case. Magnesium offers the highest negative voltage but has a low current capacity. That means it generates more electrons but lasts about a third as long as a similar-size zinc anode. In fact, magnesium can actually offer too many electrons to protect active metals such as aluminum – especially in saltwater, which is a better conductor of electricity than fresh. If you own an aluminum-hulled boat and plan to trailer it to the coast for a vacation, it's important to verify what kind of anodes it is fitted with. If they are magnesium, you want to trade them out for zinc or aluminum.
In the Great Lakes and other freshwater environments, zinc is by far the most popular anode option. That said, zinc can form an oxide film on its surface as insulation that eventually interrupts its ability to create protective electrons. You might notice this to be the case if you have zincs on your boat that doesn't seem to deteriorate.*
Anodes made by combining iridium and other metals with aluminum create alloys that don't form aluminum oxide and are another option for Great Lakes boaters. Just make sure you check all the locations where sacrificial anodes are being used. That can mean the trim tabs, prop shaft, and anode plates attached to the boat's hull, among other locations.
Cost is one consideration when selecting replacement anodes. Magnesium is more expensive than zinc, and zinc is more expensive than aluminum. There are others, too. If your boat is connected to shore power and connected to all the other boats using the dock's electrical system, zinc may be your best choice over the aluminum option to minimize the electrical activity. That said, if your boat is not plugged in or is equipped with a galvanic isolator, the more active aluminum anode might be a better choice.
I recommend talking to your local marine dealer and asking for anode advice for your particular situation. The dealer will be familiar with the local waters and what type of anodes have performed best for customers who own watercraft similar to yours. You can also call the Camp Company (727-397-6076; campcompany.com) for advice on what type of anodes you need.
When it comes to anodes, what you don't want to do is nothing, for when your boat's "zincs" have made the ultimate sacrifice and succumbed, much more expensive metals begin to take up the slack, leaving you with expensive repairs—or worse.
About the Author
Dan Armitage is a popular Great Lakes-based outdoor writer and host of the Buckeye Sportsman show (buckeyesportsman.com), syndicated weekly on 30 radio stations across Ohio. Dan is a certified Passport to Fishing instructor and leads kids fishing programs at Midwest boat and sport shows, and is a licensed Captain with a Master rating from the US Coast Guard.
A version of this article appeared in the Winter Issue (Jan/Feb) 2021 of Great Lakes Scuttlebutt magazine.
tags: Sacrificial Anodes, Winter Upkeep, Winterization
